1. Field of the Invention
This invention relates to apparatus for measuring the particulate matter content in a gaseous medium; and, more particularly, to such measuring apparatus which includes a temperature control device for permitting application both to hot gases and to cold, wet gases.
2. Description of the Prior Art
Apparatus for measuring the particulate matter content of gaseous media are significant components in many systems for monitoring and controlling particulate emissions. As acceptable levels of such emissions have been progressively lowered, the need for greater accuracy in such measuring apparatus has been accentuated. A typical prior art particulate matter measuring apparatus comprises an exhaust nozzle connected to a filter housing by an exhaust tube, a filter disposed in the housing, and an exhaust device, such as an ejector, connected to the housing. In operation, the nozzle is coupled to a duct through which a gas medium containing particulate matter, e.g., dust, passes and the exhaust device is activated. A portion of the gas medium is drawn through the nozzle, the duct, and the filter and then through the exhaust device, out of the apparatus. Particulate matter is removed from the medium by the filter. Using this apparatus, one can monitor the rate of flow of gas through the duct by measuring the quantity of gas exhausted when the suction of the exhaust device is adjusted to equate the intake velocity of the gas through the nozzle with the velocity of gas through the duct. Under these conditions, one can calculate the total amount of particulate matter carried by the gas in the duct from a measurement of the particulate matter collected by the apparatus filter.
While some devices for high or low temperature applications have been provided with air-cooled exhaust tubes and electrically heated exhaust tubes, respectively, none of the prior art devices provide a compact device suitable for both temperature extremes as well as intermediate levels likely to be encountered in operation. In apparatus for high temperature applications, e.g., for measuring the hot gases exhausted by a rotary kiln, it is necessary to prevent excessive heating of the filter. In such applications, it is known to use an air-cooled exhaust tube which is typically an exhaust tube with a jacket having passages for receiving compressed air. The compressed air cools the exhaust tube and, thereby, the hot gases therein.
In apparatus for low temperature applications, e.g., for measuring cold, wet gases, it is necessary to prevent undue condensation in the apparatus, for the condensate can trap dust before it reaches the filter and thereby interfere with accurate measurement. In such applications, it is known to use an electrically heated exhaust tube which is typically an exhaust tube provided with a jacket containing electrical heating elements.
While exhaust tubes cooled by compressed air have proved to be satisfactory when the content of dust in hot gases is to be measured, and electrically heated exhaust tubes have proved to be satisfactory when the content of dust in cold, wet gases is to be measured, it is often impractical to shift between the two different types of exhaust tubes when different conditions are encountered.